Time of flight (TOF) three-dimensional (3D) cameras determine distances to features in a scene by acquiring an image, conventionally referred to as a “range image”, of the scene that can be processed to determine how long it takes light to travel from the camera to the features and back to the camera. The round trip flight times of the light to, and back from the features, and the speed of light are used to determine the distances to the imaged features.
In a gated TOF-3D camera, light pulses are transmitted to illuminate a scene and the camera is shuttered on, “gated on”, following each light pulse for a relatively short exposure period to enable pixels in a photosensor of the camera to receive and register light from the light pulses reflected by features in the scene that reaches the camera. A pixel in the photosensor registers an amount of incident light it receives by generating, and accumulating, a quantity of electrons, hereinafter “photoelectrons”, in the pixel that is substantially proportional to the amount of incident light. The accumulated photoelectrons are transmitted to a readout pixel comprised in the photosensor. A readout circuit converts a charge provided by the photoelectrons transferred to the readout pixel to a voltage, hereinafter also “photo-voltage”, which provides a measure of the charge, and thereby the number of photoelectrons and the corresponding amount of incident light. A process of converting photoelectrons in a pixel to a photo-voltage is referred to as “reading” the pixel.
In order to provide reliable measurements of amount of light, photosensors are typically provided with a readout pixel that can accumulate a number of photoelectrons before it saturates that is equal to about twice a number of photoelectrons that saturates a pixel of the photosensor. A pixel saturates when a number of photoelectrons generated or received by the pixel is greater than the pixel's capacity to store photoelectrons. Photoelectrons in excess of a saturation threshold drain off to ground and cannot contribute to measurements of light.
Determination of a quantity of photoelectrons accumulated in a pixel by reading the pixel is subject to error from shot noise and read noise. Shot noise is a statistical error that is a function of the number of photoelectrons accumulated. If Ne is the number of photoelectrons, shot noise may be estimated to result in a fractional error (fractional error is % error multiplied by 100) in Ne equal to about 1/✓Ne. Read noise is independent of the number of photoelectrons. Read noise is a “device” noise that is introduced into a determination of a number of photoelectrons by the process of determining the number. If read noise is represented by a constant RE a fractional error in Ne may be estimated to be equal to about RE/Ne.
Distance to a feature in the scene imaged on a pixel of the camera photosurface is determined as a function of photo-voltage representing an amount of light that reaches the pixel from the feature during the short exposure periods, normalized to a photo-voltage representing a total amount of light reflected from the pulses that reaches the pixel from the feature. For convenience, light registered during the short exposure periods of the camera is referred to as “gated” light. An amount of light used to normalize gated light to determine distance is received during exposure periods that are substantially longer than the short exposure periods used to provide gated light, and is referred to as “ungated light”.
Typically, widths of the light pulses and the short exposure periods are very small, and may for example be less than or about equal to 20 nsec (nanoseconds). As a result, amounts of gated light registered by pixels in a gated TOF-3D camera are often relatively small, and errors in measurements of gated light, and consequently in distance measurements provided by the TOF-3D camera due to shot and read noise can be relatively large. For features of a scene that have low reflectivity and/or are relatively far from the camera, amounts of light from the features registered by pixels on which the features are imaged can be so small that shot noise error in measurements of the light generates unacceptably large errors in distance measurements to the features. If camera parameters and/or lighting are adjusted to compensate for low reflectivity and/or for relatively large distances of features in the scene, near and/or high reflectivity features in the scene may be poorly imaged because light from these features saturates pixels in the camera.
As a result, for many applications of a TOF-3D camera, the camera does not have a dynamic range (DNR) for measuring incident light sufficiently large to acquire range images that provide acceptable distance measurements for both “dim” and “bright” features in a scene. A camera DNR is typically measured as a ratio between a maximum amount of light per unit area of the camera photosensor, which the camera measures with an acceptable precision, divided by a minimum amount of light per unit area of the photosensor, which the camera measures with an acceptable precision.
Light incident on a surface per second per unit area of the surface is referred to as irradiance. A maximum amount of light incident per unit area of a photosensor that a camera measures with an acceptable precision may therefore be referred to as a maximum integrated irradiance, (MaxIr). Similarly, a minimum amount of light incident per unit area of a photosensor that a camera measures with an acceptable precision may be referred to as a minimum integrated irradiance, (MinIr). A camera DNR may conveniently be defined by an expression DNR=MaxIr/MinIr.